The bleaching of chemical pulp is generally carried out in a sequence, which comprises several bleaching stages with different bleaching chemicals. A usual bleaching sequence comprises a first chlorine dioxide stage (D0) followed by an alkaline bleaching stage (E) and a second chlorine dioxide stage (D1). The chlorine dioxide stages are normally carried out only with chlorine dioxide, but alternatively the chlorine dioxide can be completed with other chemicals such as ozone. The chemicals are added in different phases of the bleaching stage. The alkaline bleaching stage is normally an extraction stage, with the addition of oxygen gas and peroxide (EOP). Alternatively, the alkaline stage can be completed with an ozone phase, which is then carried out under acid conditions.
One object of the bleaching of pulp is to increase the ISO-brightness of the pulp, which implies that impurities and discoloring matter are dissolved out of the pulp and removed by washing/dewatering before and after the bleaching stages by means of filters or presses. The filtrate from the dewatering stage can, to a large, extent be re-cycled and used for washing and dilution in previous stages of the process in order to minimize the emissions of dissolved substances. For environmental reasons a low effluent volume is desired, but reflux of the filtrate causes a build-up of the substance content in the bleaching stages, with the resulting risk of increased chemical consumption.
In bleaching sequences of the aforedescribed type the chlorine dioxide stages are carried out with a pulp of mean concentration (about 8 to 15%). As it is desired prior to the first chlorine dioxide stage (D0) to remove dissolved substances from previous treatment stages to the greatest possible extent, the pulp is dewatered by pressing to high concentrations (about 25 to 40%). It is, therefore, necessary to lower the pulp concentration by dilution before the pulp is introduced into the bleaching stage. This dilution step is often carried out with clean water, but by using instead the filtrate from the washing/dewatering directly after the bleaching stage (D0) the filtrate flow, and thereby also the total effluent volume, from the process could be reduced.
This recycling of filtrate, however, has the disadvantage of deteriorating the ISO-brightness of the pulp. In order to compensate for this, increased chemical addition is required, which involves higher costs.